Aftert upgrading to Ubuntu 11.04 my server would not boot. It looks like there is a bug in the update such that if you upgrade from Ubuntu 10.10 which itself had been upgraded from Ubuntu 10.04, and grub was never reinstalled then you just get a GRUB prompt after bootup.
To fix this you need to manually boot the system using grub commands. Then reinstall grub. For me the following worked
linux /boot/vmlinux-2.6.38-8-generic root=/dev/sda1 ro
Once booted then run
sudo grub-install /dev/sda
And then it should all reboot nicely